The preparation of polychloroprene has been known for a long time and is carried out by emulsion polymerization in an alkaline aqueous medium, cf. “Ullmanns Encyclopädie der technischen Chemie”, volume 9, p. 366, Verlag Urban und Schwarzenberg, Munich-Berlin 1957; “Encyclopedia of Polymer Science and Technology”, vol. 3, p. 705-730, John Wiley, New York 1965; “Methoden der Organischen Chemie” (Houben-Weyl) XIV/1, 738 et seq. Georg Thieme Verlag Stuttgart 1961.
Possible emulsifiers are in principle all compounds and mixtures thereof which stabilize the emulsion adequately, such as e.g. the water-soluble salts, in particular the sodium, potassium and ammonium salts, of long-chain fatty acids, colophony and colophony derivatives, higher molecular weight alcohol sulfates, arylsulfonic acids, formaldehyde condensates of arylsulfonic acids, nonionic emulsifiers based on polyethylene oxide and polypropylene oxide and polymers which have an emulsifying action, such as polyvinyl alcohol (DE-A 2 307 811, DE-A 2 426 012, DE-A 2 514 666, DE-A 2 527 320, DE-A 2 755 074, DE-A 3 246 748, DE-A 1 271 405, DE-A 1 301 502, U.S. Pat. No. 2,234,215, JP-A 60-031 510.
Polychloroprene is either used for the production of industrial rubber articles, after appropriate compounding and vulcanization, or it is employed as a contact adhesive raw material (Handbook of Adhesives; chapter 21, Verlag Van Nostrand Reinhold, New York, 2nd edition 1977).
Contact adhesives based on polychloroprene are predominantly solvent-containing adhesives which are applied to both components to be joined and are allowed to dry. By subsequent joining of the two components under pressure, a bond of high strength at room temperature and, after addition of appropriate high-melting resins, also at higher temperatures (heat resistance) is obtained.
For ecological and economic reasons there is a growing demand for suitable aqueous polychloroprene dispersions which can be processed to corresponding aqueous adhesive formulations. A disadvantage here is, however, that during storage the pH of these alkaline dispersions already decreases after a short storage time, even at room temperature. This undesirable effect is accelerated at higher storage temperatures. Dispatch and storage at the customer's premises can therefore take place only in thermo-containers. Adhesive formulations prepared from these dispersions also show this change in pH during storage and must be buffered accordingly.
The addition of high-melting resins, such as e.g. magnesium chelate complexes of alkylphenol resins—which significantly increase the heat resistance of the gluings of solvent-containing adhesives—is not possible in the field of formulations of polychloroprene dispersions. The production of gluings with a high heat resistance by addition of polyisocyanates to the adhesive formulation—a method which has been known for a long time for solvent-containing polychloroprene adhesives—leads to only a moderate increase in the heat resistance of formulations based on aqueous polychloroprene dispersions, since the polyisocyanate dispersion reacts chiefly with the water molecules to give urea, and to only a small extent with the few reactive groups in the polychloroprene.
The object was therefore to provide an aqueous polychloroprene dispersion which is distinguished by a long storage stability, i.e. of which the pH does not change significantly during the storage time, which in adhesive formulations achieves a gluing of high initial strength and high heat resistance—preferably without the addition of a high-melting resin—and which has a significantly higher reactivity towards polyisocyanate dispersions.
The conditioning of polychloroprene dispersions with a high solids content is known from the prior art. EP-A 0 857 741 reports that a product with a good reactivity towards dispersed polyisocyanates is obtained by storage of the dispersion at 50° C. A disadvantage which manifests itself here is that the pH of the dispersion is lowered significantly and the electrolyte content increased significantly by this operation. Both these circumstances lower the stability during storage and during formulation to give adhesives.
The preparation of crosslinked (gel-containing) polychloroprene dispersions is furthermore known. This polymerization is described in U.S. Pat. No. 5,773,544. By a polymerization up to a high monomer conversion, gel-containing polymer dispersions which are distinguished in adhesive formulations by their high heat resistance are obtained. A disadvantage which manifests itself here also is the low storage stability of the dispersions.